I think the formula is something along the lines of :
Casualities =(unmodified casualties/1+Military Tactics), so a tactics of 1 would reduce your loses by 50%, but I could be remembering the formula for Victoria 2.
that is also the formula used in EU3 since HTTT according to the wiki. But the reason I said that it didn't have an impact till LT 18 was due to the big increases in damage at LT 18 in addition to the tactics going up to 2 at the same time.You are correct that it reduces damage by close to 50% due to roundings, but the bonus damage of calvary over infantry during the early years is greater than this reduction.